Natural Reactors
The first controlled nuclear reactor, built during World War II, was a great achievement, but it was not the first reactor to operate on planet Earth. Mother Nature put one together without the benefit of human problem-solving, which only appeared several billion years later.
A typical reactor fuel is the uranium isotope U-235. This isotope has, like all uranium atoms, 92 protons, and its atomic weight—the sum of the numbers of neutrons and protons—is 235. For better or for worse, U-235 is not found in great abundance, making up only about .7% of naturally occurring uranium, and essentially all the rest is U-238. Both U-235 and U-238 spontaneously decay, with half-lives given below.
The fission process in U-235. Note the three additional neutrons emitted, which can sustain fission if there are a sufficient number of surrounding U-235 nuclei.
- Half-life of U-235 = 7.04 x 108 years
- Half-life of U-238 = 4.47 x 109 years
With the passing of each half-life, on the average half of the remaining nuclei decay into nuclei with a lower atomic number by the emission of an alpha particle (a helium nucleus) and gamma rays. This kind of decay is not fission and cannot lead to a chain reaction.
When U-235 absorbs a neutron, there is a high probability that it will promptly fission, with the release of two to three more neutrons, as shown in the drawing. These, in a sufficiently large and concentrated sample of U-235, can set off a chain reaction and release tremendous amounts of energy. A nuclear reactor that sustains a chain reaction is said to be “critical.”
The probability of fission in U-235 depends on the speed of the neutrons, with slow neutrons being much more effective. Substances that cause neutrons to slow down are called moderators, so bathing the uranium in the moderator greatly intensifies the fission process. Water is the moderator of choice for most nuclear reactors, being plentiful and safe and having the added benefit that if the reactor gets out of control and heats up, the water boils off, shutting down the reaction.
Uranium ore contains only about .7% U-235 and is not concentrated enough to sustain a chain reaction without a moderator, so uranium ore must be enriched to a concentration of about 3% to serve as fuel for a natural reactor. In 1956, the Japanese physicist Paul Kuroda realized that, due to the difference in the half-lives of U-235 and U-238 (see above), at earlier times the relative concentration of U-235 would be higher than at present, and this might make possible a naturally-occurring fission reactor. In a remarkable piece of analysis, he detailed the necessary uranium concentration, the relative concentration of U-235/238, and even the reactor shape. Unfortunately, Karuda did not consider the possibility that porous rock might enable a natural water moderator, so he could not identify any uranium mines that matched his requirements
